除了基础类型,都可以做为通配符的边界

List<? extends String >                   
List<? extends Comparable >               
List<? extends int[] >
List<? extends Comparable<? super Long> >
List<? extends Class<? extends Number> >  
List<? extends Enum<?> >    

参考:
http://www.angelikalanger.com/GenericsFAQ/FAQSections/TypeArguments.html#Which%20types%20are%20permitted%20as%20wildcard%20bounds?